With the increasing usage of smart devices in the cockpit and always improving avionics software available for those devices, I want to design a plug-and-play, Bluetooth-enabled AHRS platform for gliders and GA aircraft.
It should be able to determine aircraft attitude in three axes, magnetic heading, geographic position via satellite navigation, pressure altitude, airspeed and current net energy. Additionally, there will be an NMEA 0183 interface to directly connect devices like FLARM without the need for separate Bluetooth adapters.
The sensor platform should be able to collect basically all data of the aircraft necessary for flight operation and safety. It needs to determine position, attitude, airspeed, altitude and total energy, receive collision avoidance data from FLARM and ADB-S receivers and transmit all that to any smart device via Bluetooth.
The first sensor is an uBlox GNSS module. This is kind of optional, as almost all modern smart devices already have an internal GPS receiver. But they tend to have a quite unreliable fix and inaccurate positioning due to their low energy budget and low gain passive internal antennas. This doesn't necessarily appear in normal applications, but when the internal compensation algorithms and plausibility checks fail due to you doing loopings after climbing a few hundred meters in a minute, issues come apparently. Also, the on-board GNSS receiver could come in handy for implementing advanced sensor fusion in the future, for example for live wind calculation.
Then there comes the heart of an AHRS, the 9-axis IMU-sensor. This comes all integrated into a single chip, combined readily with sensor fusion and calibration algorithms. That is almost too good to believe, but yes, the Bosch BNO055 claims to output ready-to-use data from the internal three-axis MEMS accelerometer, gyroscope, and magnetometer. It isn't even more expensive than other similar chips without internal sensor fusion.
Then there come the three pressure sensors. Two absolute and one differential pressure sensors. The differential sensor is there to measure the airspeed. It measures the difference between total pressure and static pressure. Total pressure is the sum of dynamic and static pressure, as it comes straight from the pitot tube. The differential sensor measures how much higher one pressure is over the other, so static pressure cancels out and leaves the component we're interested in for airspeed measurement.
Static pressure also has to be sensed by the absolute pressure sensor to get the pressure altitude, which of course can be compensated to the appropriate output by the user in software on the output device.
The second absolute pressure sensor is for an optional specialty value, the output of the "total energy probe". It is basically a reversed pitot tube mainly used on gliders to accurately compensate a variometer to show the actual change of potential and kinetic energy and not just an altitude change. The probe pneumatically subtracts dynamic pressure from static pressure. So when for example the altitude increases due to thermals, there is a discrepancy between static pressure decrease due to altitude gain and dynamic pressure decrease due to airspeed decrease. The same can also be calculated with the measured airspeed and static pressure, but due to sensor drift, this remains the most accurate method.
The interfaces consist of an NMEA 0183 interface for FLARM and ADS-B receivers and a Bluetooth module for data output to a mobile device. There might also be some additional flexible sensor inputs to sense flap and gear position, outside temperature, motor rpm and the like.
The Main processor will be an ATSAMD21G18 which is a quite powerful ARM M0+ MCU to allow for unrestrained software feature creep ;)